It is previously known in the art to employ temporally accurate activations of ammunition units at or on target areas. Among other things, it is previously known to discharge, from one and the same gun, salvos of ammunition units which are activated at the same time on the target area for efficient combatting thereof. As a result of this process, the risk will be avoided that the enemy is warned by the first round of the salvos and will have time to withdraw from the effects of subsequent rounds. In such instances, the ammunition units must be capable of being discharged at different angles of elevation of the gun, at different muzzle velocities, different cycles in the ballistic trajectories and the like. The muzzle velocity is an important parameter in this context and it is of crucial importance to maintain efficient control over this. In this context, it is previously known to measure and sense the muzzle velocity and to employ such measurements and sensing for further control and prediction of the ballistic trajectory. It is also known to employ part charges which are composed or combined in the gun so as to give different ranges and muzzle velocities (V.sub.o) from the gun. It is also known, on firing in the field, to remove a certain quantity of the powder from such part charges on site, which thus gives a loss of powder at the site of the gun.